The invention pertains to a machine for coating continuous strip materials such as sheet steel or aluminum. More particularly it pertains to the control of the thickness of a coating which is applied to the bottom face of the strip.
A well developed field exists in the art of machines for continuous coating of strip materials. A common application of this is in the area of applying paint or primer to a strip of sheet metal which is initially in the form of large coils. In a typical industrial setting, the material is uncoiled and delivered to a machine which applies the coating to both faces of the strip. After exiting the coating machine the strip progresses through a drying oven. As the wet strip cannot be handled while it progresses through the oven, the strip has the form of a catenary suspended by the coating machine at the entrance to the oven and by a coiler, take-up bridle, or other receiving device at the exit. In certain ovens a part of the weight of the strip may be supported by gas flotation.
To insure a smooth flow of the strip, the end of each coil must be attached (welded) to the beginning of the next. To allow the attachment without altering the progress of the strip through the coating machine and oven, an accumulator is provided upstream of the coating machine. This permits the line to be stopped upstream of the accumulator to attach the end of one coil to the beginning of the next, while downstream of the accumulator, movement of the strip is not interrupted. Downstream of the oven the strip may be recoiled by a coiling unit and the coils can be cut from the strip by a cutting unit. Another accumulator permits continuous movement through the oven with intermittent stops to shear the strip adjacent to the coiler.
A typical coating machine applies a coating to the top face of the strip prior to applying a coating to the bottom face. A strip enters the machine and is held between a backup roll and a top applicator roll which applies the coating to the top face of the strip. A transfer roll adjacent to the top applicator roll is partially immersed in a coating material reservoir. The thickness of the coating applied to the top face of the strip can be varied by varying the pressure between the applicator and backup rolls. Similarly the amount of coating delivered to the applicator roll can be controlled by varying the pressure between the transfer and applicator rolls. Relative speeds of the rolls and strip can also be used for control.
The strip is then routed to the bottom applicator roll which applies a coating to the bottom face of the strip. After leaving the bottom applicator roll, the strip enters the drying oven.
As the seam created by the attachment of sequential coils of strip material passes through the coating machine, it has the potential to damage the applicator rolls. To avoid this damage the top applicator roll may be disengaged from the strip by the same means that control the pressure between the two.
Since the wet strip exiting the machine is supported by the bottom applicator roll, this roll cannot be so easily disengaged. To protect the bottom applicator roll, a lift roll is included between the backup and bottom positioned below the strip and is not in contact with it. As the seam passes through the machine, the lift roll is rapidly raised and brought into contact with the bottom face of the strip. The lift roll lifts the strip out of contact with the bottom applicator roll so that the catenary is temporarily suspended from the lift roll. As soon as the seam has passed, the lift roll is lowered and the strip again is supported by the bottom applicator roll.
Since the coating on the top face of the strip is wet, no corresponding backup roll can be provided for the bottom applicator roll. Thus, the force between the bottom applicator roll and the strip is provided by tension in the strip as a function of the angle of wrap of the strip around the bottom applicator roll.
There is a tension T in the strip and the strip wraps around the roll through an angle .THETA. (defined by the arc of contact between strip and roll). If viewed with respect to a reference point midway along that arc, a section of strip coming from the backup roll and a section of strip leading to the drying oven each exert a force of a magnitude T on the bottom applicator roll. It can be seen that the circumferential components of the forces cancel each other, but the radial (normal) components do not. The radial components are each given by the expression Tsin(.THETA./2), for a total normal force N=2Tsin(.THETA./2).
If the oven has a fixed geometry, then to maintain a correspondingly fixed geometry of the catenary form of the strip, the tension in the strip and the orientation at which the strip leaves the roll must be fixed. Accordingly if the path of the strip between the backup and bottom applicator roll is also fixed, the force between the strip and the bottom applicator roll, and thus the thickness of the coating on the bottom face are fixed by the catenary geometry. It should be noted that the forces which are referred to are clearly not discrete forces operating at discrete locations, but rather net forces due to a pressure distribution along the arc of contact between the strip and the roll. Where the oven has a gas flotation system, the shape of the catenary is determined by the depth the catenary reaches due to weight of the strip as counteracted by gas flotation. The tension on the strip at the coater may be varied if the flotation pressure is varied to compensate for the tension applied by the take-up bridle (decreasing flotation to correspond with increasing tension and vice versa).
One problem that is encountered in obtaining uniform film thickness of a coating is oscillation of the strip in the drying oven. Such uncontrolled oscillations may occur in a strip supported entirely between the bottom applicator roll and a take-up bridle at the end of the oven, or may occur in a strip partly supported by flotation. When an oscillation occurs, there may be variations in the contact between the strip and the bottom applicator roll. These variations may be seen as slight streaks in the coating. It would be highly desirable to avoid such streaks.
The thickness of the coating applied to the bottom face of a strip is thus not readily controllable. For a given coating material the thickness is predetermined based on many factors including the geometry of the coating machine, viscosity of the coating material, temperature, relative speeds of various rolls, roll eccentricity, nip pressures between the coater rolls, surfaces on the rolls and the shape and weight of the catenary.
It is therefore desirable to produce a strip coating machine which features a mechanism to control the thickness of the coating that is applied to the bottom face of a strip as well as to the top face.